In 2004, a laser speckle pattern technology used in an optical mouse is published, which uses the principle of average size invariance in laser speckles to propose that the laser speckles will move as the light source moves with respect to the illuminated surface. Thereby, once the average moving velocity of the laser speckles on the detector is known, the relative movement information between the light source and the illuminated surface can be deduced. Thus, the technology can be applied to an optical mouse. Laser speckles are interference images of scattered light from an illuminated surface on a detector. The intensity of the images is related to the phase difference between the light paths of the scattered light. When the laser light source moves with respect to the illuminated surface, the phase differences of the scattered light paths changes accordingly, which result in corresponding changes in the interference images on the detector. Thereby, laser speckles are prone to distortion. Although changes in the light paths result in changes in the distribution of image intensity, the average intensity of images changes little, because it is only related to the intensity of the laser light source and to the reflectivity of the illuminated surface. When the laser light source moves with respect to the illuminated surface, said two conditions do not change, and thereby the average intensity of images barely changes. In addition, the roughness of the surface that causes the light speckles is evenly distributed, hence the average density of the laser speckles distributed on the detector should vary little, which means that the average size of the laser speckles varies little as well. The publication was filed in March 2003, and was accepted in June 2003. Thereafter, on Jan. 19, 2004 in Shenzhen of China, Chang applied the first patent named “An Optical Signal Processing Method of a Computer Mouse” using laser speckles to implement functions of a computer mouse. The patent, as the publication described above, declares that though the laser speckles distort, the average size of the laser speckles obeys the diffraction principle. Once the imaging apparatus for light speckles is fixed, the average size of the light speckles can be predicted. Chang proposed detecting the light speckles by using four detectors arranged in two perpendicular directions on a plane, with two detectors in each direction. When the laser light moves with respect to the illuminated surface the laser speckles move accordingly. By comparing the signal timing on the two horizontal and two vertical detectors caused by the light-speckle movement, the horizontal and vertical movement directions of the light speckles can be judged. Besides, in a sampling time, by multiplying the number of light speckles passing the detectors by the average size of light speckles gives the average movement distance in the sampling time. By knowing the movement directions and distance, the relative movement information between the light source and the illuminated surface can be calculated. The description above is the signal processing method provide by Chang.
The same idea is applied to the patent of a light-speckle stylus by Agilent Technologies. In April 2004, Agilent Technologies proposed the idea of adopting five detectors including top, bottom, left, right, and center sensors. The distances between the sensors are, in principle, approximately equal to the average size of light speckles. The movement information of the light speckles is given by the related timing of the light speckles passing said five sensors.
In February 2005, Agilent Technologies announced their patent of a laser mouse, which stressed that the laser speckle sizes of a normal optical pickup head are around hundreds of nanometers to several micrometers, which are too small in comparison to a sensor with 30 micrometers in pixel size. Accordingly, a sensor pixel can contain tens of light speckles, and hence the variation in signals of the light speckles is not obvious because averaging effect is easy to happen. Thereby, it is difficult to acquire effective information from the signals of the light speckles directly. Agilent Technologies emphasized that not only laser speckles are used in their patent. They proposed an optical pickup head design with specular reflection. In which light-path design taking advantage of the reflection angle being equal to the incident angle, two-dimensional sensors are set at the location where the reflection angle is equal to the incident angle. The signals of the reflection light and the scattered light are measured and analyzed to give the relative movement information between the light source and the illuminated surface.
The authors of the present invention also have filed a patent related to a laser computer mouse. The patent relates to an imaging apparatus for undistorted light speckles and a method thereof, which solve the distortion problem of laser speckles and the identification problem caused by small speckle sizes. The authors of the present invention proposed a two-dimensional imaging apparatus for light speckles by using non-specular reflection, which measure the laser speckles at the location where the scattering angle differs from the reflection angle by 10 degrees. In addition, a light-limiting device is adopted to limit the incident angular field of view at which the scattered light enters the two-dimensional detector, that is to say, to confine the imaging area on the surface. By properly combining such parameters as size of speckles, focal length of the imaging lens, angle of imaging, and imaging area of surface, an undistorted optical speckle pattern is created on the image surface. When the imaging apparatus has a relative motion with respect to the surface, an optical speckle emerges from one side of the image sensor and moves successively until it disappears on the other side of the sensor, the shape and intensity of this optical speckle remains the same. Since the speckle captured by the imaging apparatus is only shifted but distorted, it is favorite for precision pattern recognition.
The laser mouse developed by using the principle of average size invariance in light speckles as the basis for signal processing has the problem of cursor irregular jumping for some illuminated surfaces (such as glossy surfaces). This is because the glossier the illuminated surface is, the weaker the images of the light speckles are, and the smaller the sizes thereof. Thereby, the method with average size invariance can result in errors that cause instability in the results of signal processing. When this technology is applied to applications requiring precise angular resolution, for example, finger guidance, the demand cannot be satisfied.
The acquired images of light speckles by using specular reflection structure consist of two components. The majority component comes from the planar uniform reflection light; the minority component comes from the scattered light of rugged grains. The uniform reflection light has constant phase, while the scattered light of rugged grains changes in phases. These two kinds of light with distinct characteristics will interfere with each other and form interference patterns. By analyzing the correlation between two consecutive interference patterns, the relative movement information between the light source and the illuminated surface can be given. If the illuminated surface is very rough, then the amount of the uniform reflection will reduce drastically, the energy of the scattered light will increase, and the phases change rapidly, which is disadvantageous for the recognition of pattern correlation.
The imaging apparatus for light speckles and the method thereof developed previously by the authors of the present invention adopt a two-dimensional imaging apparatus which is properly combined with parameters such as size of speckles, focal length of the imaging lens, angle of imaging, and imaging area of surface. Thereby, when the light source moves with respect to the illuminated surface, the phase changes of the light speckles are limited effectively. As a result, the light-speckle patterns are resistant to distortion, which is advantageous to pattern recognition. However, because the light-limiting device is used to limit the angular field of view of the scattered light incident on the two-dimensional detector, the observation range on the surface of an object is limited to a small region. Besides, the relative moving distance between the imaging apparatus and the illuminated surface is constrained, otherwise the characteristic of undistortion in light speckles cannot be maintained ‘when the observed small region is moving, which is unfavorable for the application of the technology especially for large-area detection on the surface of the object. If the tolerable relative moving distance with light-speckle undistorted is to be widened, the distance between the imaging lens and the surface of the object has to be lengthened to compensate the negative effect of increased moving distance. Thereby, the optical read head becomes larger, which does not comply with the trend of lightness, thinness, shortness, and smallness. Hence, improvements have to be made.